Methods for forming screw threads



Dec. 5, 1967 J. w. SHIRLEY 3,355,929

METHODS FOR FORMING SCREW THREADS Filed June 8, 1965 4 Sheetsheet l F/g /a Dec. 5, 1967 J. w. SHIRLEY METHODS FOR FORMING SCREW THREADS 4 Sheets-Sheet 2 Filed June 8, 1965 Dec. 5, 1967 J. w. SHIRLEY METHODS FOR FORMING SCREW THREADS 4 Sheets-Sheet 5 Filed June 8, 1965 Dec. 5, 1967 J. w. SHIRLEY METHODS FOR FORMING SCREW THREADS 4 Sheets-Sheet 4 Filed June 8, 1965 United States Patent METHODS FOR FORMING SCREW THREADS John Wallis Shirley, Birmingham, England, assignor to Wellington Engineering Works Limited, Tipton, England, a company of Great Britain Filed June 8, 1965, Ser. No. 462,316

Claims priority, application Great Britain, June 9, 1964, i 23,771/64, 23,772/64 6 Claims. (Cl. 72-367) ABSTRACT OF THE DISCLOSURE The specification discloses a method of forming a screw thread on a tube intermediate the ends thereof by bulging a portion of the tube to form a collar or fin and then acting on the flanks of the collar or fin to form it into a discontinuous flange in the form of a helix. The specification also discloses apparatus for carrying out the method and a scaflold prop incorporating a tube made by the method.

Where a screw thread is required on a tube for only a portion of the length thereof it is often necessary to make the tube stronger over the unthreaded portion than would otherwise be necessary since sufficient thickness has to be left in the tube for the latter to be sufliciently strong at the threaded portion after the thread has been cut therein. A particular example of this problem arises in a scaffolding prop in which an inner tube is slidable in an outer tube and the latter has one end externally screw threaded to receive a collar which is used in adjusting the position of the inner tube relative to the outer tube. Thus, for example, although the load required to be taken by the outer tube could be taken by an unthreaded tube of 11 gauge (0.104 in. thick) material the outer tube is in fact made of 7 gauge (0.176 in. thick) material so that the outer tube will be suificiently strong after the external thread has been cut thereon. It is apparent that other similar situations arise.

It is one object of the present invention to provide a method and apparatus for forming a screw thread on a tube which reduces or overcomes the above disadvantage so that the size of the tube can be chosen solely with a view to taking the load required and will not be weakened by the formation of the screw thread.

It is another object of the present invention to provide a screw threaded assembly in which an external thread provided upon a tube does not reduce the strength of the tube wall.

According to the invention I provide a method of forming an external screw thread having it starts (where n is an integer greater than 0) on a tube of a material capable of plastic deformation intermediate the ends of the tube comprising the steps of compressing the tube by acting on the ends thereof to shorten the tube whilst simultaneously supporting the tube so that the material thereof in n helical zones, which are spaced around the circumference of the tube when n is greater than 1, move radially outwardly as the tube is shortened to provide it fins each having flanks extending outwardly from the remainder of the tube and subsequently compressing each fin by acting on the flanks thereof to consolidate the material therein to form the fin into a discontinuous flange in the form of a helix.

The thread may be single start or multi-start. Since there is a displacement of the material of the tube between the ends of the helical flange or between the adjacent ends of adjacent flanges it is desirable to provide in the tube, prior to compression, one or more axial slots which will, after compression, extend between the ends of the flange or between the adjacent ends of adjacent flanges. In the case of a single turn thread this slot enables the material at the ends of the flange to move relatively axially without causing excessive stress in the material of the tube. In the case of a multi-start thread each slot permits the material at each end of each flange to move relative to the material at the adjacent end of the adjacent flange. In either case, however, the flange can be formed without slotting the tube.

The method is applicable to any material capable of taking and holding a plastic deformation and may be applied both to metal tubes and to tubes of synthetic resin. When applied to metal tubes the thread may be formed with the tube either in the hot or cold state depending upon the material and thickness of the tube. Where the thread is formed on a mild steel tube the compression is preferably carried out while the tube is in a cherry red hot state.

The method according to the invention is particularly suitable for the formation of an external thread and is preferably carried out with the bore of the tube being supported against inward movement during the compression of the tube. The method is particularly applicable to the formation of an external thread due to the fact that, when a tube of an appropriate slenderness ratio is worked on, as it is axially compressed it tends to buckle outwards upon the initial compression so that the subsequent compression can form and consolidate the buckled portion into the flange. Where the slenderness ratio is not such as to cause appreciable buckling the portion will be upset to form the flange.

The method may be carried out so that the depth of the flange, radially of the tube, is between one and three times the thickness of the tube wall in its undeformed state.

The invention will now be described in detail by way of example with reference to the accompanying drawings in which:

FIGURES 1a and 1b together constitute a side elevation of apparatus for producing a screw thread on a tube by deformation thereof;

FIGURE 2 is a longitudinal section through part of the apparatus of FIGURE 1 showing the die parts and the gripping means and is a section on the line II-II of FIGURE 3;

FIGURE 3 is a section on the line III-III of FIGURE 2 showing the gripping means for the tube;

FIGURE 4 is a perspective view of the portions of a split die part;

FIGURE 5 is a perspective view of the other die part which is carried by the mandrel;

FIGURE 6 is a side elevation of the end portion of a tube showing the thread formed thereon;

FIGURE 7 is a cross section through the wall of the tube showing the forms of thread;

FIGURE 8 is a side elevationof a scaffolding prop embodying the invention; and

FIGURE 9 is a section through a modified form of apparatus for forming screw threads on short lengths of tu e.

Referring now to FIGURES 1, 2 and 3 the apparatus there shown comprises a base 10 which supports a bed 11 on uprights 12 and cross braces 13. The bed 11 is somewhat shorter than the base 10 and at one end of the base 10 there is provided a control panel 14, a. hydraulic pump 15, a motor 16 driving the pump and a hydraulic reservoir 17. The reservoir, pump and motor produce hydraulic pressure for the various hydraulic jacks which will hereinafter be described.

At the one end, the bed 11 is provided with a hydraulic jack 18 connected by lines 1 to the pump 15. The piston rod 20 of the jack 18 projects into a rectangular box-like structure 21 which is secured to the bed 11. The boxlike structure has vertical end walls 23 and 24 and vertical side walls 25 but is open at top and bottom. The piston rod 20 passes through a central aperture 26 in the end wall 23 and is pivotally connected at 27 to an extension piece 28 which has a screw threaded spigot 29 engaged in a spacer 30. An arm 31 is interposed between the extension piece 28 and the spacer 30. The spacer 30 is provided with a collar 32 and also with an externally threaded spigot 33 Which is received in the end of a mandrel 34 having a collar 35 at the end thereof. Between the collars 32 and 35 there is provided a ring-like bracket 36 which carries an arm 37 parallel to the mandrel and which in turn has a further vertictl arm 38 having a roller 39 at the end thereof. As will hereinafter be described, the roller 39 co-operates with a cam track.

The mandrel 34 has a reduced portion 40 at its free end thus providing a shoulder 41 at the end of the portion 40. The mandrel is surrounded by an outer supporting sleeve 42 which has a portion 43 which closely surrounds the mandrel and a portion 44, surrounding the reduced end portion 40 of the mandrel, which provides a clearance between the mandrel and the sleeve 42. In this clearance is located a first die part 45. The mandrel 44, the sleeve 42 and the die part 45 are all connected together by means of a pin 46.

The die part 45 is shown in more detail in FIGURE and it will be seen that is comprises a generally cylindrical portion 47 which is formed at one end with a face which is perpendicular to the longitudinal axis of the cylindrical portion 47 and is formed at the other end with a surface 48 which comprises a single turn of a helix, the ends of the surface being joined by an axial step 49. The surface 48 meets the outer surface of the part 47 at an angle somewhat less than a right angle.

Referring again to FIGURE 2 it will be seen that the open end portion of the sleeve 42 has a tapered entry 50 and that the helical surface 48 of the die part 45 lies within the cylindrical portion of the tube 42 and surrounds the reduced portion 40 of the mandrel 34. The sleeve 42 slides in a bearing member 51 which is secured in an aperture in the end wall 24 of the box-like member 21 by means of screws 52.

The box-like member 21 carries a hinged arm 53 which is hinged to the side Walls 25 thereof by means of a pin 54. Formed on the arm 53 is a cam track 55 upon which the roller 49 runs. It will be seen that as the mandrel moves to the left in FIGURE 2, the roller 39 will run along the cam track 55 and will serve to pivot the arm 53 in an anti-clockwise direction. The end of the arm 53 remote from the pivot pin 54 is provided with a bifurcated end 56 which engages with locating means as will hereinafter be described.

The other die part is indicated in FIGURE 4 and is of split form. Thus the die part is indicated generally at 57 and comprises two portions each of which has a part 58 of greater cross sectional area and a part 59 of lesser cross sectional area. The portions are arranged to fit together along their meeting faces 60 and when they do fit together with the parts 58 in register the surfaces 61 of the parts 59 form a single turn of a helix. The helical surfaces 61 are complementary to the helical surface 48 so that when material is consolidated between the helical surfaces the material is formed into a helical flange as will hereinafter be described.

The die part 57 is carried in a box-like member 62 shown in FIGURE 3 which is formed with lugs 64 bolted to channel-section beams 65 which form the bed 11. The box-like member 62 is provided with a base 66 to which is connected a hydraulic cylinder 67 whose piston rod 68 extends upwardly through an aperture in the base 66. A cross member 69 extends across the walls of the boxlike member 62 and provides a bearing for a pivot pin 70 upon which are mounted a pair of cranked levers 71 and 72. The lower end of the cranked levers are connected to toggle links 73 and 74 and the pivotal junction of these links is pivotally connected at 75 to a clevis 76 on the end of the piston rod 68.

The lever 71 is a double-armed lever having arms 77 between which the lever 72 is pivoted. At the upper end end of a lever 71 there is provided a half-bushing 78 which receives one of the portions of the split die 57 and the arm 72 carries another half-bushing 79 which carrier the other half portion of the die. The die is retained in position by means of cap screws 80 which engage in holes 81 in the parts 58 of each portion of the split die part 57.

The box-like member 62 is provided with a further cross member 82 which is provided with and open-topped slot 83 to receive a tube as will hereinafter be described.

It will be apparent from the foregoing that upon operation of the hydraulic jack 67 the die parts will open and close due to the pivoting of the levers 71 and 72 by the toggle links 73 annd 74.

The cross member 69 carries two projecting lugs 84 in which are received a locating pin 85. The pin has an enlarged head 86 against which a spring 87 abuts, the lower end of the spring engaging the lower lug 84. The enlarged head 86 carries a projecting stud 88 which forms a locating stud for a tube as Will hereinafter be described.

The lower end of the spring-loaded pin carries a cross pin 89 and is embraced by the bifurcated end 56 of the arm 53. The end 56 of the arm is above the pin 89 so that when the arm pivots in an anti-clockwise direction the pin will be retracted in a downward direction.

At the other end of the bed 11 there is provided an abutment 90 for an end of a tube indicated in chain lines at 91 in FIGURE 1. The tube 91 is gripped by gripping means comprising two opposed plates 92, one on each side of the tube, and located so that as the tube tends to move to the right in FIGURE 1 the ends of the plates are forced against the tube and prevent such movement. Guides 93 are provided to prevent the tube buckling during compression.

The apparatus is intended to form a single turn single start screw thread on a tube adjacent the end thereof. Referreing to FIGURE 6, this shows the finished product and will be described later but the tube 91 is, prior to being inserted in the machine, provided with an axial slot 96 adjacent one end thereof. As described above, this slot is provided so that, as the flange forming the thread is formed, the material which is displaced during deformation of the tube is not unduly stressed. It is however possible to deform the tube without the use of such a slot. No slot would be used, for example, where a tube was threaded to engage a coupling, otherwise leakage would take place.

Assuming that the tube 91 has been slotted as described it is then, if of mild steel locally heated to cherry red heat. The controls on the control panel 14 are operated to retract the piston of the cylinder 18 and to operate on the cylinder 67 to open the portions of the split die part 57. A tube is then dropped into position as indicated at 91 in FIGURE 1 into the guides 93 and the slot 96 is arranged to engage the stud 88 on the spring-loaded pin 85 forming the locating means. The cylinder 67 is then pressurized to close the split die part. At this stage the end of the tube will project as shown at 97 towards the mandrel 34.

The cylinder 18 is now pressurized to move the mandrel 34 towards the left as shown in FIGURE 2. The end 97 of the tube will first enter the tapered mouth 50 of the outer supporting sleeve 42 and simultaneously the reduced end portion of the mandrel 40 will enter the bore of the tube. The mandrel will proceed to move forwardly without obstruction until the end 97 of the tube engages the shoulder 41 on the mandrel. At this stage the part 44 of the outer supporting sleeve 42 will embrace the portions 59 of the split die part and prevent this opening during the succeeding compression. Further movement of the mandrel to the left will tend to compress the tube since the left-hand end thereof is against the abutment 90.

As the tube end portion is compressed it will shorten or thicken with the outward radial displacement of material so that the cross-secti0n dimension of the portion will increase. If a tube has the slenderness ratio shown in the drawings the tube end portion will buckle outwardly upon compression to form a collar or fin. As i the tube end portion buckles outwardly it will contact the inner surface of the parallel-sided part 44 of the outer supporting sleeve 42. It will then be prevented from any further outward movement. As the mandrel continues to move to the left, the buckled collar or fin of the tube will be caught between the helical surfaces 48 and 61 which will engage the flanks of the collar or fin which will be formed into a discontinuous flange in the form of a helix providing a single turn, single start screw thread. The slot 96 will have been arranged in line with the axial step 49 on the die part 45 so that, as shown in FIGURE 6, the finished thread, as indicated at 98, will begin and end at the slot 96. By providing the slot 96 in this position, undue stressing of that part of the material which would otherwise have to be deformed to the greatest extent is avoided.

If the stud 88 remained in the slot 96 during forward movement of the mandrel then it would interfere with such forward movement and therefore as the mandrel moves to the left in FIGURE 2 the roller 39 rides up the cam track 55 and the arm 53 pivots in an anticlockwise direction thus moving the spring loaded pin 85 downwardly and withdrawing it out of the Way of the advancing mandrel.

Referring now to FIGURE 7a it will be seen that the helical flange at screw thread 98' has sloping flanks and comprises two adjacent parts 99 of the tube wall which have been forced together between the helical surfaces 48 and 61. The tube wall is thus not weakened in any way by providing the screw thread as would be the case if the tube wall were made thicker and a screw thread were cut upon the tube wall.

The operation is completed by withdrawing the mandrel 34 during which the tube is gripped by the gripping means opening the die part 57 and removing the formed tube therefrom. The die part remains open to receive the next tube.

The method and apparatus described above are particularly suitable for acting upon a tube which is to be the outer tube of a scaffolding prop and such a prop is illustrated in FIGURE 8. Referring to this figure, the outer tube of the prop is indicated at 100 and is provided with a square base plate 101. Adjacent the end remote from the base plate the tube 100 is provided with a single turn external thread 102 of the form shown in FIGURE 6. The prop also comprises an inner tube 103 which slides within the outer tube 100 and is provided with a head plate 104. The inner tube is provided with a series of apertures some of which are indicated at 105 and is provided with a pin 106 which is arranged to fit into any one of the apertures, being fastened to the tube 103 by means of a ring 107 and chain 108. Engaged on the thread 102 is an internally threaded collar 109 having a pivoted operating handle 110.

The scaffolding prop is used in the normal manner, the approximate length of the prop being adjusted by inserting the pin 106 into the desired hole 105 so that the pin abuts the end of the collar 109. Final adjustment is then obtained by rotating the collar 109 relative to the outer tube 100 and this will lengthen the prop as the collar moves along the screw thread 102.

Referring now to FIGURE 9, this shows a simple arrangement for producing a screw thread on a short length of tube where this is desired. For example it may be desired to produce the thread on a short length of tube which is then welded to a longer length of tube or the component required to be threaded may itself be short.

Referring now to FIGURE 9 the die comprises a base 111 in the form of a circular disc having a central circular aperture 112 formed therein. Surrounding the aperture in the upper face of the disc is a circular recess 113 and seated in the recess is an outer supporting sleeve 114. A die part 115 is received within the outer supporting sleeve and also rests in the recess 113. The die part 115 has a helical end face indicated at 116. The inner diameter of the die part 116 is such as to receive the tube to be worked upon and the aperture 112 is of a size substantially equal to the bore of the tube, which is indicated at 117, to receive a mandrel as will hereinafter be described.

The other die part is indicated at 118 and is provided on its lower surface with a helical surface 119 which is complementary and arranged to co-operate with the helical surface 116 on the die part 115. The exteriors of the die parts 116 and 118 may be provided with longitudinally extending keyways (not shown) which receive pins passing through holes in the sleeve 114 to prevent relative rotation of the die parts 116 and 118.

A mandrel 120 is received within the die part 118 and is provided with a shoulder 121 to engage with the end tube. A collar 122 may be inserted between the upper end of the die part 118 and the head 123 of the mandrel to vary the length of the tube which is above the junction between the helical faces 116 and 119.

In operation, a length of tube 117 is placed within the die part 116 and the outer supporting tube 114 so that the lower end of the tube is supported in the recess 113.

The die part 118 is then inserted with the mandrel 120, to which it is secured by set screws, into the supporting sleeve 114. The mandrel and the base are then forced together by means of a hydraulic press or the like and the tube buckles outwardly as described above to form a screw thread.

It will be seen that the invention provides a very convenient way of forming a screw thread on a tube by deformation thereof and without weakening the tube. By this means, it is possible to use tubes of lesser gauge than has heretofore been used when it has been necessary to cut a thread in the tube.

What I claim then is:

1. A method of forming an external screw thread having 11 starts (where n is an integer greater than 0) on a tube of material capable of plastic deformation intermediate the ends of the tube comprising the steps of compressing the tube by acting on the ends thereof to shorten the tube whilst simultaneously supporting the tube so that the material thereof in n helical zones, which are spaced around the circumference of the tube when n is greater than 1, move radially outwardly as the tube is shortened to provide it fins each having flanks extending outwardly from the remainder of the tube and subseqeuntly compressing each fin by acting on the flanks thereof to consolidate the material therein to form the fin into a discontinuous flange in the form of a helix.

2. A method of forming a single start, single turn external screw thread on a tube of a material capable of plastic deformation intermediate the ends of the tube comprising the steps of compressing the tube by acting on the ends thereof to shorten the tube whilst simultaneously supporting the tube so that the metal in a helical zone moves radially outwardly as the tube is shortened to provide a collar having flanks extending outwardly from the remainder of the tube and subsequently compressing the collar by acting on the flanks thereof to consolidate the material of the collar to form it into a discontinuous flange in the form of a helix.

3. A method according to claim 2 wherein the tube is provided, prior to compression, with an axial slot which,

after compression extends between the ends of the helical flange 4. A method according to claim 3 wherein the tube is located against rotation prior to compression, by engagement of a locating member in the slot.

5. A method according to claim 1 wherein n is greater than 1 and wherein the tube is provided, prior to compression, with a plurality of axial slots each of which, after compression, extends between the adjacent ends of two adjacent flanges.

6. A method according to claim 5 wherein the tube is located against rotation prior to compression by engagement of a locating member in one of the slots.

References Cited CHARLES W. LANHAM, Primary Examiner.

CARL w. TOMLIN, Examiner.

W. L. SHEDD, Assistant Examiner. 

2. A METHOD OF FORMING A SINGLE START, SINGLE TURN EXTERNAL SCREW THREADED ON A TUBE OF A MATERIAL CAPABLE OF PLASTIC DEFORMATION INTERMEDIATE THE ENDS OF THE TUBE COMPRISING THE STEPS OF COMPRESSING THE TUBE BY ACTING ON THE ENDS THEREOF TO SHORTEN THE TUBE WHILST SIMULTANEOUSLY SUPPORTING THE TUBE SO THAT THE METAL IN A HELICAL ZONE MOVES RADIALLY OUTWARDLY AS THE TUBE IS SHORTENED TO PROVIDE A COLLAR HAVING FLANKS EXTENDING OUTWARDLY 